1. Field of the Invention
This invention relates to a method of treatment or disposal of high level wastes that result from the reprocessing of spent fuels in the nuclear fuel cycle. More particularly, the invention relates to a method of separating transplutonium elements such as Am and Cm from lanthanides in acidic solutions by solvent extraction.
2. Prior Art
High level wastes that result from the reprocessing of spent fuels in the nuclear fuel cycle contain many kinds of radioactive nuclides. Among them, transplutonium elements such as Am and Cm are very long-lived alpha-emitting nuclides and need be disposed of in a separate form from other nuclides. To this end, efforts are being undertaken to search for safe and positive methods of disposal such as nuclear transmutation and extraterrestrial ejection. To implement these methods, the step of separating small amounts of transplutonium elements from lanthanides that behave in chemically similar ways and that are contained in large quantities in high level wastes is necessary.
In known of separating transplutonium elements (hereunder sometimes referred to as "TPE") from lanthanides (hereunder sometimes referred to as "RE"), either the neutralization treatment of solutions is necessary or highly concentrated salts or harmful complex ions such as SCN.sup.- and N.sub.3.sup.- are utilized. Another known method is the TALSPEAK process which uses an extraction system consisting of di-2-ethylhexylphosphoric acid and diethylene triamine pentaacetic acid (DTPA). In this process, the pH of the solution to be treated must be adjusted to approximately 3.5 before the extracting operation is started. What is more, RE rather than TPE is extracted by this method. Since high level wastes (hereunder sometimes referred to as "HLW") typically contain RE in amounts at least 10 times as much as TPE, the method which involves the extraction of RE is by no means a preferred approach. A further problem with this method is that the post treatment of the remaining aqueous TPE solution is cumbersome.
Among the techniques that have been proposed in recent years, the use of sulfur (S) or nitrogen (N) atom containing heterocyclic compounds or crown ethers, which are the "soft" complexing agents introduced by Musikas in France, is held promising and the following data have been reported in laboratories: separation factor (D.sub.Am /D.sub.Eu)=.beta.=16.5-18.5 in a system consisting of 0.0037-0.0067M (initial concentration) HNO.sub.3/0.25 M ortho-phenanthroline+0.25M nonanoic acid/nitrobenzene; distribution ratios, D.sub.Am =1380 and D.sub.Eu =891 (i.e., .beta.=1.55) in a system consisting of 0.1M HNO.sub.3 /0.5 M ortho-phenanthroline/0.1M dibutylphosphoric acid (HDBP)/nitrobenzene, and D.sub.Am =0.331 and D.sub.Eu =0.0676 (.beta.=4.90) in a system consisting of 0.1 M HNO.sub.3 /0.5 M ortho-phenanthroline/0.001M HDBP/nitrobenzene; and .beta.=ca. 10 in a system consisting of tris 2,4,6-(2-pyridyl)!1,3,5-triazine (also referred to as TPTZ) and dinonyl naphthalenesulfonic acid (also referred to as HDNNS). In the last case, the distribution ratio of TPTZ (D.sub.TPTZ) was about 10 and its loss to the aqueous phase was substantial; in addition, HDNNS, being surface active, was very inefficient in phase separation. However, the greatest advantage of the last system is its ability to achieve separation even from a 0.125 M HNO.sub.3 solution.
A separating system consisting of a crown compound and hexachloro-cobalt-dicarbollide (also referred to as CCD) was developed in Russia. In a specific system composed of 18-crown-6 and CCD, a separation factor (.beta.) of about 3.2 was achieved for Am/Eu. If the initial concentration of Eu is 0.5 M, .beta. is about 6.0 and the intended separation can be accomplished with a system consisting of 0.5 M HNO.sub.3 /0.2 M 18-crown-6+0.3 M CCD/nitrobenzene. No case has been reported of the loss of 18-crown-6 to the aqueous phase in this extraction system. The system was implemented on an actual scale in Russia but the achieved .beta. value of 3-4 is not satisfactorily large.
In the methods by which the RE present in large quantities in HLW is extracted into the organic phase, the trace TPE of interest is left intact in the aqueous phase together with other impurities and, hence, an independent step is required to separate the TPE by extraction. A further problem is that if a pre-neutralization treatment is necessary, a large amount of alkali is added to the solution to be treated and this causes an undesirable effect in that liquid wastes containing large amounts of salts will result.
In the extraction systems that use the so far reported "soft" complexing agents or crown compounds, TPE is extracted selectively but the separation factor (.beta.) of RE and TPE is not adequately high. Further, TPTZ which is highly soluble in the aqueous phase experiences a substantial loss and, hence, is not suitable for use in the continuous treatment process. What is more, crown compounds and TPTZ are very expensive chemicals.